Monolithic Tandem Chalcopyrite-Perovskite Photovoltaic Device

ABSTRACT

Monolithic tandem chalcopyrite-perovskite photovoltaic devices and techniques for formation thereof are provided. In one aspect, a tandem photovoltaic device is provided. The tandem photovoltaic device includes a substrate; a bottom solar cell on the substrate, the bottom solar cell having a first absorber layer that includes a chalcopyrite material; and a top solar cell monolithically integrated with the bottom solar cell, the top solar cell having a second absorber layer that includes a perovskite material. A monolithic tandem photovoltaic device and method of formation thereof are also provided.

FIELD OF THE INVENTION

The present invention relates to tandem photovoltaic devices and moreparticularly, to tandem chalcopyrite (e.g., CIS or CIGS)-perovskitephotovoltaic devices and techniques for formation thereof.

BACKGROUND OF THE INVENTION

Tandem photovoltaic cells consisting of at least two absorbers withdifferent band gaps allow broader spectrum light harvesting and superiorphotovoltaic conversion efficiency as compared to single-junction solarcells. Tandem photovoltaic cells are often oriented with one solar cellon top of another. For optimal performance, the bandgap of the absorberin the top solar cell should be higher than the bandgap of the absorberin the bottom solar cell.

Two commonly employed types of tandem device are two-terminal devicesand four-terminal devices. Two-terminal tandem devices contain oneelectrode on top and one electrode on the bottom, with a tunnel junctionbetween the top and bottom solar cells of the device. Four-terminaltandem devices contain independent devices stacked on top of each other,wherein each independent device has its own top and bottom electrodes.Two-terminal tandem devices are more challenging to fabricate thanfour-terminal tandem devices because two-terminal tandem devices requirecurrent-matching between the top and bottom solar cells. Further, caremust be taken during fabrication of two-terminal tandem devices to notdamage the bottom solar cell during processing of the top solar cell.While having less strict current-matching and processing constraintsthan two-terminal devices, four-terminal devices nonetheless suffer fromsignificant resistance and optical losses due to their need for multipletransparent conductive contacts and reflection losses associated withthe additional substrates and layers.

Chalcogenide-based solar cells such as CuInSe₂ (abbreviated as “CIS”),Cu(In,Ga)(S,Se) (abbreviated as “CIGS”), and Cu₂ZnSn(S,Se)₄ (abbreviatedas “CZT(S,Se)”) have achieved their highest efficiency at relatively lowbandgap (approximately 1.15 electron volts (eV)). The use ofchalcogenide-based solar cells in a tandem device architecture howeverpresents some notable challenges. For instance, for maximum performance,chalcogenide-based solar cells require very high processing temperaturesof the absorber layer (above 450 degrees Celsius (° C.)). Thus,chalcogenide-based solar cells often cannot be used as the top solarcell in a tandem device since these high temperatures would degrade thebottom solar cell. Further, the low band gap of a chalcogenide absorbermakes chalcogenide solar cells not well-suited for use in a top cell.

Once formed, chalcogenide-based solar cells employ a p-n junction thatsignificantly deteriorates at temperatures above approximately 200° C.Thus, when a chalcogenide-based solar cell is used as the bottom cell,processing temperatures for the top cell must be kept below about 200°C. to maintain the p-n junction in the bottom cell. This requirement canbe a challenge meet when using conventional solar devices for the topsolar cell.

Therefore, techniques for integrating chalcogenide-based solar celldesigns into a tandem photovoltaic device architecture would bedesirable.

SUMMARY OF THE INVENTION

The present invention provides monolithic tandem chalcopyrite-perovskitephotovoltaic devices and techniques for formation thereof. In one aspectof the invention, a tandem photovoltaic device is provided. The tandemphotovoltaic device includes a substrate; a bottom solar cell on thesubstrate, the bottom solar cell having a first absorber layer thatincludes a chalcopyrite material; and a top solar cell monolithicallyintegrated with the bottom solar cell, the top solar cell having asecond absorber layer that includes a perovskite material.

In another aspect of the invention, a monolithic tandem photovoltaicdevice is provided. The monolithic tandem photovoltaic device includes asubstrate; a layer of electrically conductive material on the substrate;a first absorber layer on a side of the layer of electrically conductivematerial opposite the substrate, wherein the first absorber layerincludes a chalcopyrite material; a buffer layer on a side of the firstabsorber layer opposite the layer of electrically conductive material; atransparent front contact on a side of the buffer layer opposite thefirst absorber layer; a hole transporting layer on a side of thetransparent front contact opposite the buffer layer; a second absorberlayer on a side of the hole transporting layer opposite the transparentfront contact, wherein the second absorber layer includes a perovskitematerial; an electron transporting layer on a side of the secondabsorber layer opposite the hole transporting layer; and a transparenttop electrode on a side of the electron transporting layer opposite thesecond absorber layer.

In yet another aspect of the invention, a method of forming a monolithictandem photovoltaic device is provided. The method includes the stepsof: coating a substrate with a layer of electrically conductivematerial; forming a first absorber layer on a side of the layer ofelectrically conductive material opposite the substrate, wherein thefirst absorber layer includes a chalcopyrite material; forming a bufferlayer on a side of the first absorber layer opposite the layer ofelectrically conductive material; forming a transparent front contact ona side of the buffer layer opposite the first absorber layer; forming ahole transporting layer on a side of the transparent front contactopposite the buffer layer; forming a second absorber layer on a side ofthe hole transporting layer opposite the transparent front contact,wherein the second absorber layer includes a perovskite material;forming an electron transporting layer on a side of the second absorberlayer opposite the hole transporting layer; and forming a transparenttop electrode on a side of the electron transporting layer opposite thesecond absorber layer.

A more complete understanding of the present invention, as well asfurther features and advantages of the present invention, will beobtained by reference to the following detailed description anddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating an exemplary two-terminal, two-solarcell monolithic tandem photovoltaic device according to an embodiment ofthe present invention;

FIG. 2 is a diagram illustrating an exemplary methodology for forming atwo-terminal, two-solar cell monolithic tandem photovoltaic deviceaccording to an embodiment of the present invention;

FIG. 3 is a diagram illustrating a cross-sectional scanning electronmicroscope (SEM) image of a two-solar cell monolithic tandemphotovoltaic device formed according to the present techniques accordingto an embodiment of the present invention;

FIG. 4 is a diagram illustrating performance of the photovoltaic deviceof FIG. 3 according to an embodiment of the present invention; and

FIG. 5 is a diagram illustrating current voltage (J-V) curves of thephotovoltaic device of FIG. 3 according to an embodiment of the presentinvention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

As provided above, due to their high efficiency at a relatively low bandgap (about 1.15 electron volts (eV)) chalcogenide-based solar cells areideal candidates for the bottom solar cells in a tandem devicearchitecture. However, processing temperature constraints limit theoptions for what top solar cell may be formed on a chalcogenide-basedbottom solar cell. Namely, once formed, the chalcogenide-based bottomsolar cell should not be subjected to temperatures in excess of about200 degrees Celsius (° C.).

Advantageously, the present techniques leverage a new generation oflow-cost materials based on methylammonium metal (lead (Pb), tin (Sn))halide (iodide, chloride, bromide) perovskites which require lowprocessing temperatures (e.g., below 150° C. or even below 80° C.—seebelow). Perovskite solar cells can have efficiencies exceeding 15%. See,for example, Liu et al., “Efficient planar heterojunction perovskitesolar cells by vapour deposition,” Nature vol. 501, 395-398 (September2013), the contents of which are incorporated by reference as if fullyset forth herein. Beneficially, these perovskite materials have largeband gaps (1.5 eV to 2 eV). See, for example, A. Kojima et al.,“Organometal Halide Perovskites as Visible-Light Sensitizers forPhotovoltaic Cells,” Journal of the American Chemical Society, vol. 131,pp. 6050-6051, (April 2009), the contents of which are incorporated byreference as if fully set forth herein. This combination of lowprocessing temperature, high efficiency, and large band gap makeperovskite solar cells ideal partners for the top device in two-terminaltandem structures with chalcogenide solar cells such as CZTSe, CIS andCIGS with low Ga content.

As described above, tandem photovoltaic devices include a top solar celland a bottom solar cell wherein the band gap of the absorber in the topsolar cell is preferably higher than the band gap of the absorber in thebottom solar cell. In a two-terminal tandem photovoltaic device, thereis one top electrode and one bottom electrode, and a tunnel junctionbetween the top and bottom solar cells. A tandem, i.e., multi-junction,photovoltaic device architecture allows the combined two-solar cellstack to achieve high open-circuit voltages (Voc) reaching a maximumvalue of the sum of the two individual (i.e., top and bottom) cellvoltages. The total short-circuit current density produced by the tandemphotovoltaic device is limited by whichever of the individual solarcells produces the lower current density.

Provided herein are two-terminal monolithic tandem photovoltaic deviceshaving a chalcopyrite-based bottom solar cell and a perovskite-based topsolar cell. It is demonstrated below (see, for example, FIG. 4) that inaccordance with the present techniques the Voc of the present tandemdevices is indeed larger than either of the individual cells andapproaches the sum of the two individual Voc values, which therebydemonstrates the tandem concept.

FIG. 1 is a diagram illustrating an exemplary two-terminal, two-solarcell, monolithic tandem photovoltaic device 100 according to the presenttechniques. As shown in FIG. 1, the device 100 includes two solar cells,a chalcopyrite-based bottom solar cell (i.e., the absorber in the bottomsolar cell is a chalcopyrite material), and a perovskite-based top solarcell (i.e., the absorber in the top solar cell is a perovskitematerial).

Chalcopyrite absorber materials are chalcogenides which include copper(Cu), at least one of indium (In) and gallium (Ga), and at least one ofsulfur (S) and selenium (Se). Exemplary chalcopyrite materials employedherein include, but are not limited to, materials containing Cu, In, andSe—CuInSe₂—abbreviated herein as “CIS” and materials containing Cu, atleast one of In and Ga, and at least one of S andSe—Cu(In,Ga)(S,Se)—abbreviated herein as “CIGS.” As this implies, thedesignation (A,B) such as (In,Ga) and (S,Se) used herein signifies thatat least one of A and B is present, i.e., A and/or B. Chalcopyrite-basedsolar cells are described generally in Todorov et al.,“Solution-processed Cu(In,Ga)(S,Se)₂ absorber yielding a 15.2% efficientsolar cell,” Progress in Photovoltaics: Research and Applications, Vol.21, Issue 1, pgs. 82-87 (January 2013), the contents of which areincorporated by reference as if fully set forth herein.

The term “perovskite,” as used herein, refers to materials with aperovskite structure and the general formula ABX₃ (e.g., whereinA=CH₃NH₃ or NH═CHNH₃, B=lead (Pb) or tin (Sn), and X=chlorine (Cl) orbromine (Br) or iodine (I)). The perovskite structure is described anddepicted, for example, in U.S. Pat. No. 6,429,318 B1 issued to Mitzi,entitled “Layered Organic-Inorganic Perovskites Having Metal-DeficientInorganic Frameworks” (hereinafter “Mitzi”), the contents of which areincorporated by reference as if fully set forth herein. As described inMitzi, perovskites generally have an ABX₃ structure with athree-dimensional network of corner-sharing BX₆ octahedra, wherein the Bcomponent is a metal cation that can adopt an octahedral coordination ofX anions, and the A component is a cation located in the 12-foldcoordinated holes between the BX₆ octahedra. The A component can be anorganic or inorganic cation. See, for example, FIGS. 1 a and 1 b ofMitzi.

The term “monolithic tandem photovoltaic device,” as used herein, refersto a device containing two solar cells formed on single substrate ascompared, for example, to mechanically stacking two separate devices asdescribed in R. F. Service, “Perovskite Solar Cells Keep On Surging,”Science, vol. 344, (May 2014) (hereinafter “Service”). As will bedescribed in detail below, the layers of the present tandem photovoltaicdevice will be grown monolithically, layer-by-layer on a commonsubstrate. The result is two monolithically-integrated solar cellsformed on the common substrate. Further, in accordance with the presenttechniques, the monolithically-integrated solar cells will becurrent-matched to one another—see below.

Referring to FIG. 1, the chalcopyrite-based bottom solar cell includes asubstrate 102 coated with a layer 104 of an electrically conductivematerial (or optionally multiple layers represented generally by layer104), a chalcopyrite absorber layer 106 on a side of the electricallyconductive layer 104 opposite the substrate 102, a buffer layer 108 on aside of the chalcopyrite absorber layer 106 opposite the electricallyconductive layer 104, and a transparent front contact 110 on a side ofthe buffer layer 108 opposite the chalcopyrite absorber layer 106.

According to an exemplary embodiment, substrate 102 is a glass, ceramic,or metal substrate, and the electrically conductive layer 104 is formedfrom molybdenum (Mo), tungsten (W), nickel (Ni), tantalum (Ta), aluminum(Al), platinum (Pt), titanium nitride (TiN), silicon nitride (SiN), andcombinations including at least one of the foregoing materials. Forinstance, the electrically conductive layer 104 may be formed from analloy containing at least one of these materials, or alternatively froma stack of layers each of which contains at least one of thesematerials. According to an exemplary embodiment, the electricallyconductive layer 104 is coated on substrate 102 to a thickness ofgreater than about 0.1 micrometers (μm), e.g., from about 0.1 μm toabout 2.5 μm, and ranges therebetween.

As provided above, a two-terminal tandem photovoltaic device has one topelectrode and one bottom electrode. The electrically conductive layer104 will serve as the bottom electrode of device 100. In general, thevarious layers of the device 100 (beginning with the electricallyconductive layer 104) will be deposited sequentially onto the (common)substrate 102, one layer upon the next (i.e., monolithically), using acombination of vacuum-based and/or solution-based approaches. Thus, theresult will be a monolithic device structure constructed on a single,common substrate (i.e., substrate 102). Such a monolithic tandemperovskite/chalcopyrite device has not been demonstrated before.

According to an exemplary embodiment, the chalcopyrite absorber layer106 contains Cu, In, and Se (CuInSe₂)—i.e., chalcopyrite absorber layer106 is a CIS material. In that case, chalcopyrite absorber layer 106does not contain Ga or S, i.e., chalcopyrite absorber layer 106 is bothGa-free and S-free. A Ga-free and S-free CIS chalcopyrite absorber layeris ideal for use as a bottom solar cell absorber layer in the presenttandem device since it has a low bandgap (e.g., of about 1.0 eV).However, the present techniques also include embodiments wherein thechalcopyrite absorber layer 106 optionally also includes at least one ofGa and S (Cu(In,Ga)(S,Se)₂—i.e., the chalcopyrite absorber layer 106 isa CIGS material. Introducing Ga and/or S increases the bandgap of thechalcopyrite absorber layer 106. By way of example only, a CIS materialcontaining Cu, In, and Se has a bandgap of about 1.0 eV and a CIGSmaterial containing Cu, Ga, and Se has a bandgap of about 1.7 eV. Theinclusion of S (e.g., at a ratio of 1:1 with the Se) can be used tofurther raise the bandgap to about 1.9 eV. See, for example, Leisch etal., “Electrodeposited CIS-based Thin Films for PhotoelectrochemicalHydrogen Production,” 206^(th) Meeting of The Electrochemical Society(ECS) and 2004 Fall Meeting of The Electrochemical Society of Japan(ECSJ)), 3-8 Oct. 2004, Honolulu, Hi., the contents of which areincorporated by reference as if fully set forth herein. Because band gapabove 1.1 eV, especially above 1.2 eV is less desirable for a bottomdevice in tandem solar cells (see, for example, S. P. Bremner et al.,“Analysis of tandem solar cell efficiencies under AM1.5G spectrum usinga rapid flux calculation method,” Progress in Photovoltaics: Researchand Applications Vol. 16, Issue 3, pgs. 225-233 (May 2008), the contentsof which are incorporated by reference as if fully set forth herein), ina preferred embodiment the atomic Ga/In ratio, the S/Se ratio and thesum of both ratios is less than 0.2, preferably 0.1. Exemplary processesfor forming the chalcopyrite absorber layer 106 will be described indetail below.

The top solar cell and the bottom solar cell in the configuration shownin FIG. 1 are connected in series. Thus, the two cells must becurrent-matched in order to have a high-performance device.Current-matching in tandem photovoltaic devices is based in large parton the band gap of the absorbers in the individual solar cells.Advantageously, according to the present techniques the band gap of thechalcopyrite absorber layer 106 of the bottom solar cell can be tunedvia excluding Ga and/or S altogether (chalcopyrite absorber layer 106 isGa-free and/or S-free) or by including small amounts of Ga and/or S asdescribed above, in order to achieve a band gap of from about 1.0 eV toabout 1.9 eV, and ranges therebetween. As will be described in detailbelow, irrespective of the chalcopyrite absorber layer 106, the bandgapof the perovskite absorber layer 114 (see below) can also beindependently tuned to optimize the current-matching between the top andbottom solar cells.

For optimal performance, the bandgap of the top solar cell absorber in atandem device should be higher than the bandgap of the bottom solar cellabsorber. Thus, device 100 is configured such that the relatively higherbandgap absorber material (i.e., the perovskite absorber layer 114 (seebelow)) is used in the top solar cell and the relatively lower bandgapabsorber material (i.e., the chalcogenide absorber layer 106) is used inthe bottom solar cell. Perovskite materials have large bandgaps (e.g.,from about 1.5 electron volts (eV) to 3 eV, and ranges therebetween)which makes them well suited as a top solar cell absorber in a tandemdevice with a low-bandgap bottom cell absorber such as a chalcopyrite(e.g., about 1.0 eV with a Ga-free, S-free CIS absorber—see above).

As highlighted above, using a chalcogenide bottom solar cellconfiguration presents notable fabrication challenges. For instance,chalcopyrite solar cells may only be used as the bottom solar cell in atwo-terminal tandem device when the processing temperature of the topsolar cell remains below about 200° C. due to the instability of thechalcopyrite solar cell p-n junction above this temperature.Advantageously, exemplary techniques are implemented herein forlow-temperature perovskite formation (see below) to enable a perovskiteabsorber-based top solar cell to be produced over a chalcopyrite bottomsolar cell without damaging the bottom solar cell.

A p-n junction is formed between the buffer layer 108 and thechalcopyrite absorber layer 106. According to an exemplary embodiment,the buffer layer 108 is formed from at least one of cadmium sulfide(CdS), a cadmium-zinc-sulfur material of the formula Cd_(1-x)Zn_(x)S(wherein 0<x≦1), indium sulfide (In₂S₃), zinc oxide, zinc oxysulfide(e.g., a Zn(O,S) or Zn(O,S,OH) material), and aluminum oxide (Al₂O₃),and has a thickness of from about 50 angstroms (Å) to about 1,000 Å, andranges therebetween. According to an exemplary embodiment, thetransparent front contact 110 is formed from a transparent conductiveoxide (TCO) such as indium-tin-oxide (ITO) and/or aluminum (Al)-dopedzinc oxide (ZnO) (AZO).

As its base, the perovskite-based top solar cell has a bottom electrodewhich, according to an exemplary embodiment, is formed from a TCO suchas ITO and/or AZO (the same materials present in the transparent frontcontact 110). Thus, in the exemplary monolithic embodiment depicted inFIG. 1, the transparent front contact 110 serves as both a top electrodeof the chalcopyrite-based bottom solar cell and the bottom electrode ofthe perovskite-based top solar cell. By comparison, if instead of usingthe present techniques to monolithically grow the layers of the deviceone were to mechanically stack one solar cell on top of another, then anadditional layer(s) would be present to accommodate for a separatebottom electrode for the top solar cell. Adding additional layers to thestack can disadvantageously serve to block some of the light fromreaching the bottom solar cell. See, for example, Service.

As shown in FIG. 1, the perovskite-based top solar cell has a holetransporting layer 112 on a side of the transparent front contact 110opposite the buffer layer 108, a perovskite absorber layer 114 on a sideof the hole transporting layer 112 opposite the transparent frontcontact 110, an electron transporting layer 116 on a side of theperovskite absorber layer 114 opposite the hole transporting layer 112,and a transparent top electrode 118 on a side of the electrontransporting layer 116 opposite the perovskite absorber layer 114.

According to an exemplary embodiment, the hole transporting layer 112 isformed from a hole-transporting material such aspoly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) ormolybdenum trioxide (MoO₃). As is known in the art, electron holes (orsimply “holes”) represent the lack or absence of an electron. Electronsand holes are charge carriers in semiconductor materials.

An exemplary process for forming the perovskite absorber layer 114 willbe described in detail below. In general however, the process involvessynthesizing the perovskite absorber layer 114 from a metal halide filmand a source of methylammonium halide vapor. Advantageously, theexemplary perovskite synthesis process employed herein may be carriedout under a vacuum in order to lower processing temperatures—therebypreventing any potential p-n junction damage to the underlyingchalcopyrite-based bottom solar cell. See above.

As provided above, bandgap tuning of the perovskite absorber layer 114provides another mechanism by which current-matching between the top andbottom solar cells can be optimized. Bandgap tuning in the perovskiteabsorber layer 114 can be achieved by varying the halide concentrationin the perovskite. This bandgap tuning process for perovskites isdescribed in detail below.

According to an exemplary embodiment, the electron transporting layer116 is formed from at least one of phenyl-C61-butyric acid methyl ester(PCBM), C60 (Buckminsterfullerene), and bathocuproine (BCP). PCBM, C60,and BCP are hole-blocking, electron-transporting materials.

Transparent top electrode 118 will serve as the top electrode of the(e.g., two-terminal) device 100. According to an exemplary embodiment,transparent top electrode 118 is formed from a thin layer of metal(e.g., a layer of aluminum having a thickness of from about 5 nanometers(nm) to about 50 nm, and ranges therebetween), ITO, AZO, a silvernanowire mesh, or any other material which is both partially transparentin the visible spectrum and electrically conducting. Transparentelectrically conductive silver nanowire films are described, forexample, in Liu et al., “Silver nanowire-based transparent, flexible,and conductive thin film,” Nanoscale Research Letters, 6:75 (January2011) (hereinafter “Liu”), the contents of which are incorporated byreference as if fully set forth herein. As shown, for example, in FIG. 2of Liu, silver nanowires form a web-like film which is porous and alsotransparent.

Device 100 is a two-terminal, monolithic tandem photovoltaic device.Two-terminal means that only two electrodes of the device 100 need to becontacted: one on top, and one on the bottom (for example, layer 104 ofelectrically conductive material and transparent top electrode 118 inFIG. 1, or the aluminum on top and the molybdenum on the bottom in FIG.3—described below). The “electrodes” in the middle which complete thechalcopyrite-based bottom solar cell and begin the perovskite-based topsolar cell (e.g., ITO or AZO/PEDOT:PSS) act as a “tunnel junction” suchthat electrons from the bottom solar cell move “upwards” and recombinewith holes in the top solar cell that are moving “downwards” (i.e.,effectively what is left as current from the device is holes from thebottom cell moving “downwards” and electrons from the top cell moving“upwards”).

An exemplary process for forming the present tandem photovoltaic devicesis now described by way of reference to FIG. 2. Specifically, FIG. 2 isa diagram illustrating an exemplary methodology 200 for forming atwo-terminal, two-solar cell monolithic tandem photovoltaic device, suchas device 100 of FIG. 1. For consistency, the reference numerals fromFIG. 1 will be used here as well to refer to the same layers of thephotovoltaic device. As highlighted above, the present device is formedmonolithically layer-by-layer from the bottom up on a common substratewith the result being a chalcopyrite-based bottom solar cell and acurrent-matched perovskite-based top solar cell formed on the commonsubstrate. Accordingly, the starting platform for the fabricationprocess is substrate 102 which in step 202 is coated with a layer(s) ofan electrically conductive material 104. As provided above, suitablesubstrates include, but are not limited to, glass, ceramic, metal foil,or plastic substrates. Suitable electrically conductive materials forelectrically conductive layer 104 include, but are not limited to, Mo,Ni, Ta, W, Al, Pt, TiN, SiN, and combinations including at least one ofthe foregoing materials (for example as an alloy of one or more of thesematerials or as a stack of multiple layers of these materials). By wayof example only, electrically conductive layer 104 can be formed on thesubstrate 102 using evaporation or sputtering.

In step 204, the chalcopyrite absorber layer 106 is formed on a side ofthe electrically conductive layer 104 opposite the substrate 102. Asprovided above, in one exemplary embodiment the chalcopyrite absorberlayer 106 contains Cu, In, and Se (CuInSe₂)—i.e., chalcopyrite absorberlayer 106 is a CIS material. In that case, chalcopyrite absorber layer106 is both Ga-free and S-free (which can be beneficial for the bottomsolar cell absorber layer in the present tandem device since Ga-free andS-free CIS has a low bandgap (e.g., of about 1.0 eV)). In anotherexemplary embodiment the chalcopyrite absorber layer 106 optionallyfurther includes (in addition to Cu, In, and Se) at least one of Ga andS (Cu(In,Ga)(S,Se)₂)—i.e., the chalcopyrite absorber layer 106 is a CIGSmaterial.

Chalcopyrite absorber layer 106 may be formed using any suitablevacuum-based or solution-based approach. By way of example only,suitable deposition techniques include, but are not limited to, vapordeposition, coevaporation, physical vapor deposition (PVD) (i.e.,sputtering), etc.

Formation of a CIS material using co-evaporation is described, forexample, in AbuShama et al., “Properties of ZnO/CdS/CuInSe₂ Solar Cellswith Improved Performance,” Prog. Photovolt: Res. Appl. 2004; 12:39-45(published January 2004), the contents of which are incorporated byreference as if fully set forth herein, and Takayuki Negami et al.,“Large-area CIGS absorbers prepared by physical vapor deposition,” SolarEnergy Materials and Solar Cells, volume 67, Issues 1-4, pgs. 1-9 (March2001), the contents of which are incorporated by reference as if fullyset forth herein, and sputtering of a CIGS material is described, forexample, in U.S. Pat. No. 8,586,457 issued to Liang et al., entitled“Method of fabricating high efficiency CIGS solar cells,” (hereinafter“U.S. Pat. No. 8,586,457”), the contents of which are incorporated byreference as if fully set forth herein. In U.S. Pat. No. 8,586,457 a Cu,In, and Ga-containing precursor film is deposited by sputtering frombinary Cu—Ga and In sputter targets. Alternatively, the chalcopyriteabsorber may be formed as a stack of layers wherein the sequence of thelayers in the stack is configured so as to achieve optimal band gradingand/or adhesion to the substrate 102, as is known in the art. See, forexample, Dullweber et al., “Back surface band gap gradings inCu(In,Ga)Se₂ solar cells,” Thin Solid Films, vol. 387, 11-13 (May 2001),the contents of which are incorporated by reference as if fully setforth herein. Solution-based deposition processes are described, forexample, in U.S. Pat. No. 8,613,973 issued to Mitzi et al., entitled“Photovoltaic device with solution-processed chalcogenide absorberlayer,” the contents of which are incorporated by reference as if fullyset forth herein, and in W. Liu et al., “12% Efficiency CuIn(Se,S)₂Photovoltaic Device Prepared Using a Hydrazine Solution Process,” Chem.Mater., 2010, 22(3), pp. 1010-1014 (published August 2009) (hereinafter“Liu”), the contents of which are incorporated by reference as if fullyset forth herein.

For optimal performance, the bandgap of the top solar cell absorber in atandem device should be higher than the bandgap of the bottom solar cellabsorber. In order to achieve a low bandgap best suited for the bottomsolar cell absorber, the chalcopyrite absorber layer 106 according to apreferred embodiment is both Ga-free and S-free. As described above, theinclusion of Ga and/or S in the chalcopyrite absorber layer 106increases the bandgap. Namely, as described above, for optimalperformance the bandgap of the bottom solar cell absorber should beconfigured to be lower than the bandgap of the top solar cell absorberin order to current-match the solar cells. With a perovskite-based topsolar cell absorber having a bandgap of from about 1.5 eV to 3 eV, andranges therebetween, a bottom solar cell absorber having a bandgap ofabout 1.0 eV—as is achieved with a Ga-free and S-free CISmaterial—suitable current-matched is achieved. However, including Gaand/or S (i.e., CIGS) in the chalcopyrite absorber layer 106 can beused, if so desired, to increase the bandgap of the chalcopyriteabsorber layer 106 relative to the bandgap of the perovskite top solarcell absorber. As provided above, the bandgap of the chalcopyriteabsorber layer 106 can be adjusted (using G and/or S) to from about 1.0eV to about 1.9 eV, and ranges therebetween. Having the ability toindependently tune the bandgap of the top and bottom solar cellabsorbers is one advantageous feature of the present techniques.

As is known in the art, either during or following deposition of thechalcopyrite absorber layer materials an anneal in achalcogen-containing environment is preferably performed. Especially inlow-temperature deposition approaches such as sputtering or ink-based,as-deposited materials have poor grain structure and defects. An annealin a chalcogen environment improves the grain structure and defectlandscape in the material. The concentration and type of chalcogensources, i.e., sulfur (if present in the material) and/or seleniumsources such as but not limited to S, Se, H₂S and H₂Se can be furtherused to control the final S/Se ratio in the material and thereforecontrol the final bandgap.

In step 206, the buffer layer 108 is formed on a side of thechalcopyrite absorber layer 106 opposite the electrically conductivelayer 104. As provided above, suitable materials for forming bufferlayer 108 include, but are not limited to, at least one of CdS, acadmium-zinc-sulfur material of the formula Cd_(1-x)Zn_(x)S (wherein0<x≦1), In₂S₃, zinc oxide, zinc oxysulfide (e.g., a Zn(O,S) orZn(O,S,OH) material), and Al₂O₃. According to an exemplary embodiment,the buffer layer 108 is formed on the chalcopyrite absorber layer 106using standard chemical bath deposition.

In step 208, the transparent front contact 110 is formed on a side ofthe buffer layer 108 opposite the chalcopyrite absorber layer 106. Asprovided above, suitable materials for forming the transparent frontcontact 110 include, but are not limited to, a transparent conductiveoxide (TCO) such as ITO and/or AZO. According to an exemplaryembodiment, the transparent front contact 110 is formed on the bufferlayer 108 by sputtering. As provided above, the transparent frontcontact 110 serves as both a top electrode of the chalcopyrite-basedbottom solar cell and the bottom electrode of the perovskite-based topsolar cell in the present monolithically integrated tandem solar celldesign. Thus, formation of the transparent front contact 110 on thebuffer layer 108 completes fabrication of the chalcopyrite-based bottomsolar cell, and is the first step in fabricating the perovskite-basedtop solar cell.

The next step in forming the perovskite-based top solar cell is todeposit the hole transporting layer 112 on a side of the transparentfront contact 110 opposite the buffer layer 108. This is performed instep 210. As provided above, suitable materials for forming the holetransporting layer 112 include, but are not limited to, PEDOT:PSS, MoO₃,or any other hole-transporting material that makes ohmic contact to theperovskite. According to an exemplary embodiment, the hole transportinglayer 112 is deposited onto the transparent front contact 110 using aspin-coating or evaporation process.

As highlighted above, in a two-terminal, tandem photovoltaic device a“tunnel junction” is needed between the top and bottom electrodes thatfacilitates recombination between electrons from the bottom solar celland holes from the top solar cell. The transparent front contact110/hole transporting layer 112 (e.g., ITO or AZO/PEDOT:PSS) junction isthe tunnel junction in this case, as holes transported through the holetransporting layer 112 recombine with electrons transported through thetransparent front contact 110.

In step 212, the perovskite absorber layer 114 is formed on a side ofthe hole transporting layer 112 opposite the transparent front contact110. According to an exemplary embodiment, the perovskite absorber layer114 is formed on the hole transporting layer 112 using the techniquesdescribed in U.S. patent application Ser. No. 14/449,420, designated asAttorney Docket Number YOR920140171US1, entitled “Techniques forPerovskite Layer Crystallization” (hereinafter “U.S. patent applicationSer. No. 14/449,420”) the contents of which are incorporated byreference as if fully set forth herein. In general, the techniquesdescribed in U.S. patent application Ser. No. 14/449,420 involve usingvacuum annealing of a metal halide (e.g., lead or tin iodide, chlorideor bromide) and a methylammonium halide source (e.g., methylammoniumiodide, methylammonium bromide, and methylammonium chloride) to create amethylammonium halide vapor which reacts with the metal halide to form aperovskite material. One notable advantage of the techniques presentedin U.S. patent application Ser. No. 14/449,420 is that annealing under avacuum permits significantly lower reaction temperatures than those usedin other processes. For instance, temperatures below 150° C., e.g., fromabout 60° to about 150° C., and ranges therebetween can be employed. Asdescribed above, once formed, a chalcopyrite-based bottom solar cellshould not be subjected to processing temperatures exceeding about 200°C. so as to prevent degradation of the p-n junction.

Another notable advantage of the techniques presented in U.S. patentapplication Ser. No. 14/449,420 is that they provide for optionalreal-time monitoring of the reaction to optimize the properties of theperovskite based on the changing optical properties of the reactants asthe perovskite is being formed. Reaction and monitoring set-ups arepresented in U.S. patent application Ser. No. 14/449,420 that permitreal-time transmission (in the case of transparent samples) andreflective (in the case of non-transparent samples) measurements to bemade.

Yet another notable advantage of the techniques presented in U.S. patentapplication Ser. No. 14/449,420 is that they permit formation ofhigh-quality, uniform perovskite layers over large device areas.Specifically, the methylammonium halide source may be a methylammoniumhalide-coated substrate placed facing, and in close proximity to, themetal halide during the vacuum annealing. This configuration permitsuniform perovskite formation on large device substrates.

As provided above, the bandgap of the bottom solar cell absorber and thebandgap of the top solar cell absorber can be independently tuned. Thiscapability advantageously ensures proper current-matching between thetop and bottom solar cells. Bandgap tuning of the bottom cellchalcopyrite absorber was described in detail above. With regard to theperovskite absorber, the bandgap can be varied by varying the metalhalide composition in the perovskite. For instance, in the exemplaryperovskite formation process described above the starting metal halidelayer (which reacts with the methylammonium vapor) has the formula MX₂,wherein M is lead (Pb) and/or tin (Sn), and X is at least one offluorine (F), chlorine (Cl), bromine (Br), and/or iodine (I). Lead-andtin-based perovskite materials have different bandgaps. For instance, alead-free perovskite CH₃NH₃SnI₃ has a bandgap of 1.23 eV while apure-lead perovskite CH₃NH₃PbI₃ has a bandgap of about 1.55 eV. Changingthe halide composition of the perovskite can also affect bandgap. Forexample, the material CH₃NH₃PbBr₃ has a band gap of about 2.25 eV. Theoptimum bandgap for the top solar cell in the presentchalcopyrite/perovskite-based tandem device is about 1.7 eV for a bottomsolar cell band gap of about 1.0 eV (e.g., which is achievable using aGa-free and S-free CIS absorber—see above). A perovskite absorberbandgap of about 1.7 eV could be achieved by slightly increasing thebandgap of the CH₃NH₃PbI₃ (1.55 eV) via the introduction of Cl or Br.Alternatively, a perovskite absorber bandgap of 1.7 eV could be achievedby starting with CH₃NH₃SnI₃ (1.23 eV) and adding significantly more Clor Br. The effects of adding Cl and/or Br to a pure iodide perovskitesample are illustrated in U.S. patent application Ser. No. 14/449,486,designated as Attorney Docket Number YOR920140230US1, entitled “TandemKesterite-Perovskite Photovoltaic Device” (hereinafter “U.S. patentapplication Ser. No. 14/449,486”) the contents of which are incorporatedby reference as if fully set forth herein. Specifically, U.S. patentapplication Ser. No. 14/449,486 provides metal halide samples containingiodide alone (pure iodide), iodide/chloride, iodide/bromide, andiodide/chloride/bromide which illustrate a transition from darker tolighter as the chlorine and bromine are added to the pure-iodideperovskite. The lighter coloration of the samples is due to a reductionin light absorption to the larger bandgap of the material.

Next, in step 214 electron transporting layer 116 is formed on a side ofthe perovskite absorber layer 114 opposite the hole transporting layer112. As provided above, suitable materials for forming the electrontransporting layer 116 include, but are not limited to, at least one ofPCBM, C60, and BCP. According to an exemplary embodiment, the electrontransporting layer 116 is deposited onto the perovskite absorber layer114 using a spin-coating or evaporation process.

Finally, in step 216 the transparent top electrode 118 is formed on aside of the electron transporting layer 116 opposite the perovskiteabsorber layer 114. As provided above, suitable materials for formingthe transparent top electrode 118 include, but are not limited to, athin layer of metal (e.g., aluminum (Al)), ITO, AZO, or a silvernanowire mesh. A thin aluminum layer can be formed using evaporation.ITO and AZO are often deposited using sputtering or a chemical vapordeposition (CVD)-based process. Alternatively, a layer of ITO or AZOnanoparticles could be coated from a suspension on top of the device.Silver nanowire meshes can be prepared using various solution-basedprocesses such as spray-coating from a suspension in alcohol. Formationof the transparent top electrode 118 completes the device.

The present techniques are further described by way of reference to thefollowing non-limiting example:

A CIS (i.e., Ga-free and S-free) solar cell was prepared (similarly tothe process described in Liu) as follows: a solution of 1.683 grams (g)In₂Se₃ and 0.680 g Se in 25 milliliters (ml) hydrazine was mixed with asolution of 0.389 g Cu and 0.289 g S in 4 ml hydrazine. Five layers ofthe mixed solution were spin-coated on a molybdenum-coated glasssubstrate at 600 revolutions per minute (RPM) and annealed on a hotplate set at 540° C. A buffer layer (in this example CdS) was thendeposited onto the CZT(S,Se) to form a p-n junction, and then a ZnO/ITObilayer electrode was sputtered on top to complete the bottom solarcell.

A layer of PEDOT was spin-coated on top of the bottom solar cell at3,000 RPM and annealed at 140° C. for 15 minutes. A layer of perovskitewas deposited according to the process described in U.S. patentapplication Ser. No. 14/449,420. PbI₂ layers were prepared byspin-coating 0.8 molar (M) PbI₂ in Dimethylformamide (DMF) at 2,000 RPM.The sample was placed in the annealing apparatus described in U.S.patent application Ser. No. 14/449,420 in the presence of a close-spacedmethylammonium iodide source and treated at 80° C. for 12 hours. A layerof 2% PCBM was coated on top of the perovskite layer and a thin aluminumlayer (sheet resistance 200-1,000 ohm square and approximately 30%transmission) was deposited on top.

FIG. 3 shows a cross-sectional scanning electron microscope (SEM) image300 of the monolithic tandem chalcopyrite/perovskite device formed inthe above example. Each layer in the stack is identified on the rightside of the image. An open circuit voltage (Voc) of 1047 millivolts (mV)was measured for this device (see FIG. 4), effectively demonstrating atandem two-terminal photovoltaic device of a chalcopyrite (Vocapproximately 329 mV) and a perovskite (Voc approximately 709 mV) solarcell. FIG. 4 illustrates further characteristics of the device,including efficiency (Eff), fill factor (FF), and short circuit currentdensity (Jsc). A current voltage (J-V) curve of the tandem device ofFIG. 3, along with a reference CIS device and a reference perovskitedevice are shown in FIG. 5.

Although illustrative embodiments of the present invention have beendescribed herein, it is to be understood that the invention is notlimited to those precise embodiments, and that various other changes andmodifications may be made by one skilled in the art without departingfrom the scope of the invention.

What is claimed is:
 1. A tandem photovoltaic device, comprising: asubstrate; a bottom solar cell on the substrate, the bottom solar cellhaving a first absorber layer comprising a chalcopyrite material; and atop solar cell monolithically integrated with the bottom solar cell, thetop solar cell having a second absorber layer comprising a perovskitematerial.
 2. The tandem photovoltaic device of claim 1, wherein thechalcopyrite material comprises copper, indium, and selenium.
 3. Thetandem photovoltaic device of claim 2, wherein the chalcopyrite materialis gallium-free.
 4. The tandem photovoltaic device of claim 2, whereinthe chalcopyrite material further comprises: at least one of gallium andsulfur.
 5. The tandem photovoltaic device of claim 1, wherein thesubstrate comprises a glass, ceramic, metal foil, or plastic substrate.6. The tandem photovoltaic device of claim 1, wherein the bottom cellfurther comprises: a layer of electrically conductive material on thesubstrate, wherein the first absorber layer is present on a side of thelayer of electrically conductive material opposite the substrate; abuffer layer on a side of the first absorber layer opposite the layer ofelectrically conductive material; and a transparent front contact on aside of the buffer layer opposite the first absorber layer.
 7. Thetandem photovoltaic device of claim 6, wherein the layer of electricallyconductive material is formed from a material selected from the groupconsisting of molybdenum, nickel, tantalum, tungsten, aluminum,platinum, titanium nitride, silicon nitride, and combinations comprisingat least one of the foregoing materials.
 8. The tandem photovoltaicdevice of claim 6, wherein the buffer layer comprises at least one ofcadmium sulfide, a cadmium-zinc-sulfur material, indium sulfide, zincoxide, zinc oxysulfide, and aluminum oxide.
 9. The tandem photovoltaicdevice of claim 6, wherein the transparent front contact is formed fromindium-tin-oxide or aluminum-doped zinc oxide.
 10. The tandemphotovoltaic device of claim 1, wherein the top solar cell furthercomprises: a bottom electrode; a hole transporting layer on the bottomelectrode, wherein the second absorber layer is present on a side of thehole transporting layer opposite the bottom electrode; an electrontransporting layer on a side of the second absorber layer opposite thehole transporting layer; and a transparent top electrode on a side ofthe electron transporting layer opposite the second absorber layer. 11.The tandem photovoltaic device of claim 10, wherein the bottom electrodeis formed from indium-tin-oxide or aluminum-doped zinc oxide.
 12. Thetandem photovoltaic device of claim 10, wherein a transparent frontcontact of the bottom solar cell serves as the bottom electrode of thetop solar cell.
 13. The tandem photovoltaic device of claim 10, whereinthe hole transporting layer comprisespoly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) or molybdenumtrioxide.
 14. The tandem photovoltaic device of claim 1, wherein theperovskite material has a formula ABX₃, wherein A=CH₃NH₃ or NH═CHNH₃,B=lead or tin, and X=chlorine, bromine, or iodine.
 15. The tandemphotovoltaic device of claim 10, wherein the electron transporting layeris formed from at least one of phenyl-C61-butyric acid methyl ester,C60, and bathocuproine.
 16. The tandem photovoltaic device of claim 10,wherein the transparent top electrode is formed from a metal,indium-tin-oxide, aluminum-doped zinc oxide, or a silver nanowire mesh.17. A monolithic tandem photovoltaic device, comprising: a substrate; alayer of electrically conductive material on the substrate; a firstabsorber layer on a side of the layer of electrically conductivematerial opposite the substrate, wherein the first absorber layercomprises a chalcopyrite material; a buffer layer on a side of the firstabsorber layer opposite the layer of electrically conductive material; atransparent front contact on a side of the buffer layer opposite thefirst absorber layer; a hole transporting layer on a side of thetransparent front contact opposite the buffer layer; a second absorberlayer on a side of the hole transporting layer opposite the transparentfront contact, wherein the second absorber layer comprises a perovskitematerial; an electron transporting layer on a side of the secondabsorber layer opposite the hole transporting layer; and a transparenttop electrode on a side of the electron transporting layer opposite thesecond absorber layer.
 18. A method of forming a monolithic tandemphotovoltaic device, the method comprising the steps of: coating asubstrate with a layer of electrically conductive material; forming afirst absorber layer on a side of the layer of electrically conductivematerial opposite the substrate, wherein the first absorber layercomprises a chalcopyrite material; forming a buffer layer on a side ofthe first absorber layer opposite the layer of electrically conductivematerial; forming a transparent front contact on a side of the bufferlayer opposite the first absorber layer; forming a hole transportinglayer on a side of the transparent front contact opposite the bufferlayer; forming a second absorber layer on a side of the holetransporting layer opposite the transparent front contact, wherein thesecond absorber layer comprises a perovskite material; forming anelectron transporting layer on a side of the second absorber layeropposite the hole transporting layer; and forming a transparent topelectrode on a side of the electron transporting layer opposite thesecond absorber layer.
 19. The method of claim 18, wherein thechalcopyrite material comprises copper, indium, and selenium, andwherein the chalcopyrite material is gallium-free.
 20. The method ofclaim 18, wherein the second absorber layer is formed at a temperatureof from about 60° C. to about 150° C., and ranges therebetween.